While commercial flow cytometers enable high-throughput particle analysis with impressive sensitivity, they have limited sensitivity for analyzing small particles. The detection limit for viruses by light scattering is well below the existing sensitivity, and the detection of certain types of bacteria has proven to be difficult. Furthermore, integration with other microfluidic-based sample extraction and preparation chips remains difficult. The goal of this project is to develop a fundamentally new approach for flow cytometry that is based on measuring the mass of biological components as they flow through a suspended microchannel resonator (SMR). In conventional flow cytometry, fluorescently labeled affinity molecules (e.g. antibodies) make specific components (e.g. cells) brighter than the background ones, and the number of fluorescently labeled components is determined with optical readout. In the proposed approach, nanoparticle-affinity conjugates will make specific components heavier than the background ones, and the number of nanoparticle labeled components will be determined by weighing them with the SMR. We have recently demonstrated that the SMR can measure mass in solution with ~1 femtogram resolution in a flow-through format. Such a resolution represents a six order of magnitude improvement over a high-end commercial quartz crystal microbalance. Alternatively, one femtogram is equivalent to the mass of a 45 nm gold nanoparticle, or one percent of the mass of an E. coli bacterium in solution. The overall goal of this proposal is to develop aggregation assays for specific viruses and cells, and to validate the performance of the SMR for quantifying the degree of aggregation and hence target concentration. We envision that the nanoscale resolution of the SMR coupled with its robustness, low-cost, and microfluidic format will make mass-based flow cytometry a useful tool for the research environment.